5 50 ECIIINODERMATA ECHINOIDEA chap. 



The animal is about the size of a small potato, and is of light 

 straw colour. Its outline is oval, and the test is about two- 

 thirds as high as the shorter diameter. It is thus higher in 

 proportion to its width than is the case with any living Cake- 

 urchin. The highest point is behind the centre. The narrower 

 end of the animal terminates in a vertical edge, in the upper part 

 of which is a large periproct covered with a number of 

 thin movable plates. The mouth is situated on the under 

 surface, considerably nearer the front end of the test than the 

 hinder end. It is entirely devoid of jaws or of teeth, and also 

 of gills or of a movable peristome. 



Aristotle's lantern has entirely disappeared, leaving as the 

 only trace of its former presence a canal with membranous 

 walls encircling the mouth, which has the form of a transverse 

 slit, the posterior lip projecting considerably forv/ard. 



The ambulacral areas are easily distinguishable from the 

 interambulacral areas by being comparatively bare of spines. 

 On the upper surface they are distinctly grooved, the groove 

 being especially deep in the case of the anterior one. On the 

 lower surface they coalesce round the mouth, shvitting out the 

 interambulacral regions, and are here perforated by the large 

 pores of the buccal tube-feet. Between the two posterior radii 

 on the oral surface there is a space with specially arranged spines 

 called the plastron or sternum. The interambulacral plates com- 

 posing this region are very much lengthened, and interdigitate 

 with one another at the sutures. To this lengthening is due 

 the apparent forward shift of the mouth. The spines are very 

 characteristic, and are very different from any which have as yet 

 been described. They are the sole organs of locomotion. The 

 primaries are long and curved, with flattened tips, admirably 

 adapted to plough through the sand in which the animal lives. 

 On the upper surface, mingled with the tube-feet, are a large 

 number of small secondary spines. Between the two posterior 

 petals there is a hoop-shaped band of very small black spines. 

 These spines are ciliated, and draw a current of fresh sea-water 

 over the respiratory tube-feet. Beneath the periproct there is 

 a similar band called the " sub-anal fasciole " ; this probably 

 produces a current of water which sweeps away the material 

 ejected from the anus. 



The pedicellariae are of the trifoliate and gemmiform varieties. 



